ASTM C1402-04(2009)
(Guide)Standard Guide for High-Resolution Gamma-Ray Spectrometry of Soil Samples
Standard Guide for High-Resolution Gamma-Ray Spectrometry of Soil Samples
SIGNIFICANCE AND USE
Gamma-ray spectrometry of soil samples is used to identify and quantify certain gamma-ray emitting radionuclides. Use of a germanium semiconductor detector is necessary for high-resolution gamma-ray measurements.
Much of the data acquisition and analysis can be automated with the use of commercially available systems that include both hardware and software. For a general description of the typical hardware in more detail than discussed in Section 6, see Ref (19).
Both qualitative and quantitative analyses may be performed using the same measurement data.
The procedures described in this guide may be used for a wide variety of activity levels, from natural background levels and fallout-type problems, to determining the effectiveness of cleanup efforts after a spill or an industrial accident, to tracing contamination at older production sites, where wastes were purposely disposed of in soil. In some cases, the combination of radionuclide identities and concentration ratios can be used to determine the source of the radioactive materials.
Collecting samples and bringing them to a data acquisition system for analysis may be used as the primary method to detect deposition of radionuclides in soil. For obtaining a representative set of samples that cover a particular area, see Practice C 998. Soil can also be measured by taking the data acquisition system to the field and measuring the soil in place (in situ). In situ measurement techniques are not discussed in this guide.
SCOPE
1.1 This guide covers the identification and quantitative determination of gamma-ray emitting radionuclides in soil samples by means of gamma-ray spectrometry. It is applicable to nuclides emitting gamma rays with an approximate energy range of 20 to 2000 keV. For typical gamma-ray spectrometry systems and sample types, activity levels of about 5 Bq (135 pCi) are measured easily for most nuclides, and activity levels as low as 0.1 Bq (2.7 pCi) can be measured for many nuclides. It is not applicable to radionuclides that emit no gamma rays such as the pure beta-emitting radionuclides hydrogen-3, carbon-14, strontium-90, and becquerel quantities of most transuranics. This guide does not address the in situ measurement techniques, where soil is analyzed in place without sampling. Guidance for in situ techniques can be found in Ref (1) and (2). This guide also does not discuss methods for determining lower limits of detection. Such discussions can be found in Refs (3), (4), (5), and (6).
1.2 This guide can be used for either quantitative or relative determinations. For quantitative assay, the results are expressed in terms of absolute activities or activity concentrations of the radionuclides found to be present. This guide may also be used for qualitative identification of the gamma-ray emitting radionuclides in soil without attempting to quantify their activities. It can also be used to only determine their level of activities relative to each other but not in an absolute sense. General information on radioactivity and its measurement may be found in Refs (7), (8), (9), (10), and (11) and Standard Test Methods E 181. Information on specific applications of gamma-ray spectrometry is also available in Refs (12) or (13). Practice D 3649 may be a valuable source of information.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 This standard may involve hazardous material, operations, and equipment. This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: C1402 − 04 (Reapproved 2009)
Standard Guide for
High-Resolution Gamma-Ray Spectrometry of Soil Samples
This standard is issued under the fixed designation C1402; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope address all of the safety concerns, if any, associated with its
use. It is the responsibility of the user of this standard to
1.1 This guide covers the identification and quantitative
establish appropriate safety and health practices and deter-
determination of gamma-ray emitting radionuclides in soil
mine the applicability of regulatory limitations prior to use.
samples by means of gamma-ray spectrometry. It is applicable
to nuclides emitting gamma rays with an approximate energy
2. Referenced Documents
range of 20 to 2000 keV. For typical gamma-ray spectrometry
2.1 ASTM Standards:
systems and sample types, activity levels of about 5 Bq (135
C998 Practice for Sampling Surface Soil for Radionuclides
pCi) are measured easily for most nuclides, and activity levels
C999 Practice for Soil Sample Preparation for the Determi-
as low as 0.1 Bq (2.7 pCi) can be measured for many nuclides.
nation of Radionuclides
It is not applicable to radionuclides that emit no gamma rays
C1009 Guide for Establishing a Quality Assurance Program
such as the pure beta-emitting radionuclides hydrogen-3,
forAnalytical Chemistry Laboratories Within the Nuclear
carbon-14, strontium-90, and becquerel quantities of most
Industry
transuranics. This guide does not address the in situ measure-
D3649 PracticeforHigh-ResolutionGamma-RaySpectrom-
ment techniques, where soil is analyzed in place without
etry of Water
sampling. Guidance for in situ techniques can be found in Ref
E181 Test Methods for Detector Calibration andAnalysis of
(1) and (2). This guide also does not discuss methods for
Radionuclides
determining lower limits of detection. Such discussions can be
IEEE/ASTM-SI-10 Standard for Use of the International
found in Refs (3), (4), (5), and (6).
System of Units (SI) the Modern Metric System
1.2 This guide can be used for either quantitative or relative
2.2 ANSI Standards:
determinations.Forquantitativeassay,theresultsareexpressed
N13.30 Performance Criteria for Radiobioassay
in terms of absolute activities or activity concentrations of the
N42.14 Calibration and Use of Germanium Spectrometers
radionuclides found to be present.This guide may also be used
for the Measurement of Gamma-Ray Emission Rates of
for qualitative identification of the gamma-ray emitting radio-
Radionuclides
nuclides in soil without attempting to quantify their activities.
N42.23 Measurement Quality Assurance for Radioassay
It can also be used to only determine their level of activities
Laboratories
relative to each other but not in an absolute sense. General
ANSI/IEEE-645 Test Procedures for High Purity Germa-
information on radioactivity and its measurement may be
nium Detectors for Ionizing Radiation
found in Refs (7), (8), (9), (10), and (11) and Standard Test
MethodsE181.Informationonspecificapplicationsofgamma-
3. Summary of Guide
ray spectrometry is also available in Refs (12) or (13). Practice
3.1 High-resolution germanium detectors and multichannel
D3649 may be a valuable source of information.
analyzers are used to ensure the identification of the gamma-
1.3 The values stated in SI units are to be regarded as
ray emitting radionuclides that are present and to provide the
standard. No other units of measurement are included in this
best possible accuracy for quantitative activity determinations.
standard.
3.2 For qualitative radionuclide identifications, the system
1.4 This standard may involve hazardous material,
must be energy calibrated. For quantitative determinations, the
operations, and equipment. This standard does not purport to
system must also be shape and efficiency calibrated. The
This guide is under the jurisdiction ofASTM Committee C26 on Nuclear Fuel
Cycle and is the direct responsibility of Subcommittee C26.05 on Methods of Test. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
CurrenteditionapprovedJune1,2009.PublishedJuly2009.Originallyapproved contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
in 1998. Last previous edition approved in 2004 as C1402 – 04. DOI: 10.1520/ Standards volume information, refer to the standard’s Document Summary page on
C1402-04R09. the ASTM website.
The boldface numbers in parentheses refer to the list of references at the end of Available fromAmerican National Standards Institute (ANSI), 25 W. 43rd St.,
this standard. 4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
C1402 − 04 (2009)
standard sample/detector geometries must be established as sissoftwarecandeconvolutemultipletswheretheseparationof
part of the efficiency calibration procedure. any two adjacent peaks is more than 0.5 FWHM (see Refs (20)
and (21)). For peak separations that are smaller than 0.5
3.3 The soil samples typically need to be pretreated (for
FWHM, most interference situations can be resolved with the
example, dried), weighed, and placed in a standard container.
use of automatic interference correction algorithms (22).
Forquantitativemeasurements,thedimensionsofthecontainer
holding the sample and its placement in front of the detector 5.2 Ifthenuclidesarepresentinthemixtureinveryunequal
must match one of the efficiency-calibrated geometries. If radioactive portions and if nuclides of higher gamma-ray
multiple geometries can be selected, the geometry chosen energy are predominant, the interpretation of minor, less
should reflect the detection limit and count rate limitations of energetic gamma-ray photopeaks becomes difficult due to the
the system. Qualitative measurements may be performed in high Compton continuum and backscatter.
non-calibrated geometries.
5.3 True coincidence summing (also called cascade sum-
3.4 The identification of the radionuclides present is based ming) occurs regardless of the overall count rate for any
on matching the energies of the observed gamma rays in the radionuclide that emits two or more gamma rays in coinci-
spectrum to computer-based libraries of literature references dence. Cobalt-60 is an example where both a 1173-keV and a
[see Refs (14), (15), (16), (17),or (18)]. The quantitative 1332-keV gamma ray are emitted from a single decay. If the
determinations are based on comparisons of observed count sample is placed close to the detector, there is a finite
rates to previously obtained counting efficiency versus energy probability that both gamma rays from each decay interact
calibration data, and published branching ratios for the radio- within the resolving time of the detector resulting in a loss of
nuclides identified. counts from both full energy peaks. Coincidence summing and
the resulting losses to the photopeak areas can be considerable
4. Significance and Use (>10 %)beforeasumpeakatanenergyequaltothesumofthe
coincident gamma-ray energies becomes visible. Coincidence
4.1 Gamma-ray spectrometry of soil samples is used to
summing and the resulting losses to the two individual photo-
identify and quantify certain gamma-ray emitting radionu-
peak areas can be reduced to the point of being negligible by
clides. Use of a germanium semiconductor detector is neces-
increasing the source to detector distance or by using a small
sary for high-resolution gamma-ray measurements.
detector. Coincidence summing can be a severe problem if a
4.2 Much of the data acquisition and analysis can be
well-type detector is used. See Test Methods E181 and (7) for
automated with the use of commercially available systems that
more information.
include both hardware and software. For a general description
5.4 Random summing is a function of count rate (not dead
ofthetypicalhardwareinmoredetailthandiscussedinSection
time) and occurs in all measurements. The random summing
6, see Ref (19).
rate is proportional to the total count squared and to the
4.3 Both qualitative and quantitative analyses may be per-
resolving time of the detector and electronics. For most
formed using the same measurement data.
systems, uncorrected random summing losses can be held to
less than1%by limiting the total counting rate to less than
4.4 The procedures described in this guide may be used for
1000 count/s. However, high-precision analyses can be per-
a wide variety of activity levels, from natural background
formed at high count rates by the use of pileup rejection
levels and fallout-type problems, to determining the effective-
circuitry and dead-time correction techniques. Refer to Test
ness of cleanup efforts after a spill or an industrial accident, to
Methods E181 for more information.
tracing contamination at older production sites, where wastes
were purposely disposed of in soil. In some cases, the
6. Apparatus
combination of radionuclide identities and concentration ratios
can be used to determine the source of the radioactive 6.1 Germanium Detector Assembly—The detector should
materials. have an active volume of greater than 50 cm , with a full width
at one half the peak maximum (FWHM) less than 2.0 keV for
4.5 Collecting samples and bringing them to a data acqui-
the cobalt-60 gamma ray at 1332 keV, certified by the
sition system for analysis may be used as the primary method
manufacturer. A charge-sensitive preamplifier should be an
to detect deposition of radionuclides in soil. For obtaining a
integral part of the detector assembly.
representative set of samples that cover a particular area, see
Practice C998. Soil can also be measured by taking the data 6.2 Sample Holder Assembly—As reproducibility of results
acquisition system to the field and measuring the soil in place
depends directly on reproducibility of geometry, the system
(in situ). In situ measurement techniques are not discussed in should be equipped with a sample holder that will permit using
this guide.
reproducible sample/detector geometries for all sample con-
tainer types that are expected to be used at several different
5. Interferences sample-to-detector distances.
5.1 In complex mixtures of gamma-ray emitters, the degree 6.3 Shield—The detector assembly should be surrounded by
of interference of one nuclide in the determination of another a radiation shield made of material of high atomic number
isgovernedbyseveralfactors.Interferencewilloccurwhenthe providingtheequivalentattenuationof100mm(ormoreinthe
photopeaks from two separate nuclides overlap within the case of high background radiation) of low-activity lead. It is
resolutionofthegamma-rayspectrometer.Mostmodernanaly- desirable that the inner walls of the shield be at least 125 mm
C1402 − 04 (2009)
distant from the detector surfaces to reduce backscatter and ties. In addition to the analysis capabilities, it is important to
annihilationradiation.Iftheshieldismadeofleadorhasalead consider the overall user interface and architecture of the
liner, the shield should have a graded inner shield of appropri- software. For small-scale operations, a few samples per week,
ate materials, for example, 1.6 mm of cadmium or tin-lined a user interface that requires a lot of user intervention is
with 0.4 mm of copper, to attenuate the induced 88-keV lead sufficient. For larger-scale operations, with hundreds of
fluorescent X-rays. The shield should have a door or port for samples per week on multiple detectors, a software package
inserting and removing samples. The materials used to con- that permits some kind of batch processing and automated
struct the shield should be prescreened to ensure that they are operation is recommended.
not contaminated with unacceptable levels of natural or man-
7. Container for a Test Sample
made radionuclides.The lower the desired detection capability,
themoreimportantitistoreducethebackground.Forverylow
7.1 Sampleholdersandcontainersmusthaveareproducible
activity samples, the detector assembly itself, including the geometry.Considerationsincludecommercialavailability,ease
preamplifer, should be made of carefully selected low back-
of use and disposal, and the containment of radioactivity for
ground materials. protection of the working environment, personnel, and the
gamma-ray spectrometer from contamination. For small soil
6.4 High-Voltage Power/Bias Supply—The bias supply re-
samples (up to a few grams), plastic bottles are convenient
quired for germanium detectors usually provides a voltage up
containers, while large samples (up to several kilograms),
to 65000 V and 1 to 100 µA. The power supply should be
which require greater sensitivity, are frequently packaged in
regulatedto0.1 %witharippleofnotmorethan0.01 %.Noise
Marinelli beakers. For analyzing low-energy gamma rays at
caused by other equipment should be removed with r-f filters
close geometries, the consistency of the wall thickness of the
and power line regulators.
sample container facing the detector becomes an important
6.5 Amplifier—A spectroscopy amplifier which is compat-
factor in the variability of the analysis results.
ible with the preamplifier. If used at high count rates, a model
7.2 Measurements may require precautions to prevent the
with pile-up rejection should be used. The amplifier should be
loss of volatile radionuclides. For example, the direct determi-
pole-zeroed properly prior to use.
nation of radium-226 in soil by the measurement of the
6.6 Data Acquisition Equipment—A multichannel pulse- 609-keV gamma ray of bismuth-214 assumes secular equilib-
height analyzer (MCA) with a built-in or stand-alone analog- rium between radium-226 and its bismuth-214 progency and
to-digital converter (ADC) compatible with the amplifier that the radon-222 daughter was not lost from the sample.
output and pileup rejection scheme. The MCA(hardwired or a
7.3 Abeta absorber consisting of about 6 mm of aluminum,
computer-software-based) collects the data, provides a visual
beryllium, or plastic should be placed between the detector and
display, and stores and processes the gamma-ray spectral data.
sample for samples that have significant quantities of high-
The four major components of an MCA are: ADC, memory,
energy beta emitters.
control, and input/output. TheADC digitizes the analog pulses
fromtheamplifier.Theheightofthesepulsesrepresentsenergy
8. Calibration and Standardization
deposited in the detector.The digital resu
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation:C1402–98 Designation:C1402–04 (Reapproved 2009)
Standard Guide for
High-Resolution Gamma-Ray Spectrometry of Soil Samples
This standard is issued under the fixed designation C 1402; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide covers the identification and quantitative determination of gamma-ray emitting radionuclides in soil samples by
means of gamma-ray spectrometry. It is applicable to nuclides emitting gamma rays with energies greater than an approximate
energy range of 20 to 2000 keV. For typical gamma-ray spectrometry systems and sample types, activity levels of about 5 Bq (135
pCi) are measured easily for most nuclides, and activity levels as low as 0.1 Bq (2.7 pCi) can be measured for many nuclides. It
is not applicable to radionuclides that emit no gamma rays such as the pure beta-emitting radionuclides hydrogen-3, carbon-14,
strontium-90,andbecquerelquantitiesofmosttransuranics.Thisguidedoesnotaddresstheinsitumeasurementtechniques,where
soil is analyzed in place without sampling. Guidance for in situ techniques can be found in Ref (1) and (2). This guide also does
not discuss methods for determining lower limits of detection. Such discussions can be found in Refs (3), (4), (5) , and (6).
1.2 This guide can be used for either quantitative or relative determinations. For quantitative assay, the results are expressed in
terms of absolute activities or activity concentrations of the radionuclides found to be present. This guide may also be used for
qualitative identification of the gamma-ray emitting radionuclides in soil without attempting to quantify their activities. It can also
be used to only determine their level of activities relative to each other but not in an absolute sense. General information on
radioactivity and its measurement may be found in Refs (7), (8), (9), (10) , and (11) and General Methods E-181and StandardTest
Methods E 181. Information on specific applications of gamma-ray spectrometry is also available in Refs (12) or (13). Practice
D 3649 is a valuable source of information.
1.3may be a valuable source of information.
1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.4 This standard may involve hazardous material, operations, and equipment. This standard does not purport to address all
of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate
safety and health practices and determine the applicability of regulatory limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
C 998 Practice for Sampling Surface Soil for Radionuclides
C 999 Practice for Soil Sample Preparation for the Determination of Radionuclides
C 1009 GuideforEstablishingaQualityAssuranceProgramforAnalyticalChemistryLaboratoriesWithintheNuclearIndustry
D 3649 Practice for High-Resolution Gamma-Ray Spectrometry of Water
E 181 GeneralTest Methods for Detector Calibration and Analysis of Radionuclides
EI380EEE/ASTM-SI-10 PracticeStandard for Use of the International System of Units (SI) the ModernizedModern Metric
System
2.2 ANSI Standards:
N13.30 Performance Criteria for Radiobioassay
N42.14 Calibration and Use of Germanium Spectrometers for the Measurement of Gamma-Ray Emission Rates of
Radionuclides
N42.23 Measurement Quality Assurance for Radioassay Laboratories
ANSI/IEEE-645 Test Procedures for High Purity Germanium Detectors for Ionizing Radiation
This guide is under the jurisdiction ofASTM Committee C–26C26 on Nuclear Fuel Cycle and is the direct responsibility of Subcommittee C26.05 on Methods of Test.
Current edition approved July 10, 1998. Published December 1998.
Current edition approved June 1, 2009. Published July 2009. Originally approved in 1998. Last previous edition approved in 2004 as C 1402 – 04.
The boldface numbers in parentheses refer to the list of references at the end of this standard.
For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
, Vol 12.01.volume information, refer to the standard’s Document Summary page on the ASTM website.
Annual Book of ASTM Standards, Vol 11.02.
Available from American National Standards Institute (ANSI), 25 W. 43rd St., 4th Floor, New York, NY 10036, http://www.ansi.org.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
C1402–04 (2009)
3. Summary of Guide
3.1 High-resolution germanium detectors and multichannel analyzers are used to ensure the identification of the gamma-ray
emitting radionuclides that are present and to provide the best possible accuracy for quantitative activity determinations.
3.2 For qualitative radionuclide identifications, the system must be energy calibrated. For quantitative determinations, the
system must also be shape and efficiency calibrated. The standard sample/detector geometries must be established as part of the
efficiency calibration procedure.
3.3 The soil samples typically need to be pretreated (for example, dried), weighed, and placed in a standard container. For
quantitative measurements, the dimensions of the container holding the sample and its placement in front of the detector must
match one of the efficiency-calibrated geometries. If multiple geometries can be selected, the geometry chosen should reflect the
detection limit and count rate limitations of the system. Qualitative measurements may be performed in non-calibrated geometries.
3.4 Theidentificationoftheradionuclidespresentisbasedonmatchingtheenergiesoftheobservedgammaraysinthespectrum
to computer-based libraries of literature references (see[see Refs (14), (15), (16), (17),or (18)]. The quantitative determinations
are based on comparisons of observed count rates to previously obtained counting efficiency versus energy calibration data, and
published branching ratios for the radionuclides identified.
4. Significance and Use
4.1 Gamma-ray spectrometry of soil samples is used to identify and quantify certain gamma-ray emitting radionuclides. Use of
a germanium semiconductor detector is necessary for high-resolution gamma-ray measurements.
4.2 Muchofthedataacquisitionandanalysiscanbeautomatedwiththeuseofcommerciallyavailablesystemsthatincludeboth
hardware and software. For a general description of the typical hardware in more detail than discussed in Section 6, see Ref (19).
4.3 Both qualitative and quantitative analyses may be performed using the same measurement data.
4.4 The procedures described in this guide may be used for a wide variety of activity levels, from natural background levels
and fallout-type problems, to determining the effectiveness of cleanup efforts after a spill or an industrial accident, to tracing
contamination at older production sites, where wastes were purposely disposed of in soil. In some cases, the combination of
radionuclide identities and concentration ratios can be used to determine the source of the radioactive materials.
4.5 Collecting samples and bringing them to a data acquisition system for analysis may be used as the primary method to detect
deposition of radionuclides in soil. For obtaining a representative set of samples that cover a particular area, see Practice C 998.
Soil can also be measured by taking the data acquisition system to the field and measuring the soil in place (in situ). In situ
measurement techniques are not discussed in this guide.
5. Interferences
5.1 In complex mixtures of gamma-ray emitters, the degree of interference of one nuclide in the determination of another is
governed by several factors. Interference will occur when the photopeaks from two separate nuclides overlap within the resolution
of the gamma-ray spectrometer. Most modern analysis software can deconvolute multiplets where the separation of any two
adjacent peaks is more than 0.5 FWHM (see Refs (20) and (21) ). For peak separations that are smaller than 0.5 FWHM, most
interference situations can be resolved with the use of automatic interference correction algorithms (22).
5.2 If the nuclides are present in the mixture in very unequal radioactive portions and if nuclides of higher gamma-ray energy
are predominant, the interpretation of minor, less energetic gamma-ray photopeaks becomes difficult due to the high Compton
continuum and backscatter.
5.3 True coincidence summing (also called cascade summing) occurs regardless of the overall count rate for any radionuclide
that emits two or more gamma rays in coincidence. Cobalt-60 is an example where both a 1173-keV and a 1332-keV gamma ray
are emitted from a single decay. If the sample is placed close to the detector, there is a finite probability that both gamma rays from
each decay interact within the resolving time of the detector resulting in a loss of counts from both full energy peaks. Coincidence
summing and the resulting losses to the photopeak areas can be considerable (>10 %) before a sum peak at an energy equal to the
sum of the coincident gamma-ray energies becomes visible. Coincidence summing and the resulting losses to the two individual
photopeak areas can be reduced to the point of being negligible by increasing the source to detector distance or by using a small
detector. Coincidence summing can be a severe problem if a well-type detector is used. See GeneralTest Methods E 181 and (7)
for more information.
5.4 Random summing is a function of count rate (not dead time) and occurs in all measurements. The random summing rate
is proportional to the total count squared and to the resolving time of the detector and electronics. For most systems, uncorrected
random summing losses can be held to less than1%by limiting the total counting rate to less than 1000 count/s. However,
high-precision analyses can be performed at high count rates by the use of pileup rejection circuitry and dead-time correction
techniques. Refer to GeneralTest Methods E 181 for more information.
6. Apparatus
6.1 Germanium Detector Assembly —The detector should have an active volume of greater than 50 cm , with a full width at
one half the peak maximum (FWHM) less than 2.0 keV for the cobalt-60 gamma ray at 1332 keV, certified by the manufacturer.
A charge-sensitive preamplifier using low-noise field-effect transistors should be an integral part of the detector assembly.
6.2 Sample Holder Assembly—As reproducibility of results depends directly on reproducibility of geometry, the system should
C1402–04 (2009)
be equipped with a sample holder that will permit using reproducible sample/detector geometries for all sample container types
that are expected to be used at several different sample-to-detector distances.
6.3 Shield—The detector assembly should be surrounded by a radiation shield made of material of high atomic number
providing the equivalent attenuation of 100 mm (or more in the case of high background radiation) of low-activity lead. It is
desirable that the inner walls of the shield be at least 125 mm distant from the detector surfaces to reduce backscatter and
annihilation radiation. If the shield is made of lead or has a lead liner, the shield should have a graded inner shield of appropriate
materials, for example, 1.6 mm of cadmium or tin-lined with 0.4 mm of copper, to attenuate the induced 88-keV lead fluorescent
X-rays.Theshieldshouldhaveadoororportforinsertingandremovingsamples.Thematerialsusedtoconstructtheshieldshould
be prescreened to ensure that they are not contaminated with unacceptable levels of natural or man-made radionuclides.The lower
the desired detection capability, the more important it is to reduce the background. For very low activity samples, the detector
assembly itself, including the preamplifer, should be made of carefully selected low background materials.
6.4 High-Voltage Power/Bias Supply —The bias supply required for germanium detectors usually provides a voltage up to
65000 V and 1 to 100 µA. The power supply should be regulated to 0.1 % with a ripple of not more than 0.01 %. Noise caused
by other equipment should be removed with r-f filters and power line regulators.
6.5 Amplifier—A spectroscopy amplifier which is compatible with the preamplifier. If used at high count rates, a model with
pile-up rejection should be used. The amplifier should be pole-zeroed properly prior to use.
6.6 DataAcquisition Equipment —Amultichannel pulse-height analyzer (MCA) with a built-in or stand-alone analog-to-digital
converter (ADC) compatible with the amplifier output and pileup rejection scheme.The MCA(hardwired or a computer-software-
based) collects the data, provides a visual display, and stores and processes the gamma-ray spectral data. The four major
componentsofanMCAare:ADC,memory,control,andinput/output.TheADCdigitizestheanalogpulsesfromtheamplifier.The
heightofthesepulsesrepresentsenergydepositedinthedetector.ThedigitalresultisusedbytheMCAtoselectamemorylocation
(channel number) which is used to store the number of events which have occurred at the energy. The MCAmust also be able to
extend the data collection time for the amount of time that the system is dead while processing pulses (live time correction).
6.7 Data Output Equipment—Modern MCAs provide a wide range of input and output (I/O) capabilities. Typically, these
include the ability to transfer any section of data to one or more of the following: terminal, line printer, cassette tape, floppy or
hard disk, X-Y plotter, and to computer interfaces by means of a serial or parallel port or Ethernet connection.
6.8Count Rate Meter— It is useful but not mandatory to have a means to measure the total count rate for pulses above the
amplifier noise during the measurement. If not provided by the MCA, a separate count rate meter may be used for this purpose.
Intheabsenceofaratemeter,countratesthataretoohightoprovidereliableresultsmayalsobedetectedbymonitoringthesystem
dead time or peak resolution, or both.
6.9
6.8 Pulser—Required only if random summing effects are corrected with the use of a stable pulser (23) and (24) .
6.10
6.9 Computer—Mostmoderngamma-rayspectrom
...
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